A box contains $11$ red marbles, $6$ green marbles, and $3$ blue marbles. If a marble is randomly chosen, what is the probability that it is not red?
Explanation: There are $11 + 6 + 3 = 20$ marbles in the box. There are $11$ red marbles. That means $20 - 11 = 9$ are not red. The probability is $ \frac{9}{20}$.